Watermarking of multimedia content (still imagery, motion imagery, or audio) is the process of modifying the digital content in order to embed information into the content and the corresponding process of recovering that information from the modified content. One example of such watermark information is a digital forensic code added or embedded to digital multimedia content, such as digital audio or digital video, after production and before or during distribution. In this case, the watermark or digital forensic code is intended to apply a unique identifier to each of many copies of a multimedia work that are otherwise identical. In one application, this can be used to identify the source of an illegally copied digital multimedia item. Watermarking digital content of multimedia applications, such as digital video and digital audio, is one technique to deter thieves from misappropriating a copy of the work and then illegally redistributing it. This technique also encourages authorized distributors of digital content to maintain high security standards because watermarking can identify the specific authorized dealer from which the misappropriated copy originated. For example, if an illegal copy of digital content is confiscated, the watermark information within the digital content can be used to determine the identity of the authorized distributor and, perhaps, the time and place of the public showing of the digital content by the authorized distributor via the use of serial numbers in the forensic code. With this information, an investigation can begin at the identified authorized distributor to determine the conditions under which the misappropriation occurred.
In many applications, a unit of digitally watermarked content may undergo some modification between the time it is embedded and the time it is detected. These modifications are named “attacks” because they generally degrade the watermark and render its detection more difficult. If the attack is expected to occur naturally during the process of authorized or unauthorized distribution, then, the attack is considered “non-intentional”. Examples of non-intentional attacks can be: (1) a watermarked image that is cropped, scaled, JPEG compressed, filtered etc. (2) a watermarked digital product that is converted to NTSC/PAL SECAM for viewing on a television display, MPEG or DIVX compressed, re-sampled etc. On the other hand, if the attack is deliberately done with the intention of thwarting the purpose of the watermark, then the attack is “intentional”, and the party performing the attack is a thief or pirate. The three classes of intentional attack are unauthorized embedding, unauthorized detection, and unauthorized removal. This invention is concerned with unauthorized removal; removing the watermark or impairing its detection (i.e. the watermark is still in the content but cannot be easily retrieved by the detector). Unauthorized removal attacks generally have the goal of making the watermark unreadable while minimizing the perceptual damage to the content. Examples of attacks can be small, imperceptible combinations of line removals/additions and/or local rotation/scaling applied to the content to make difficult its synchronization with the detector (many watermark detectors are sensitive to de-synchronization).
One type of attack is a collusion attack where different copies of the same content are combined in an attempt to disguise or scramble the different watermark information contained in each. It would be useful to develop a technique to accurately retrieve the digital watermark information from pirated digital content where collusion has attempted to disrupt the watermark information.